People have long speculated that changes in climate can affect human society. For instance, many ancient civilizations, including the Mayan, are thought to have collapsed due to drought.

Now, a study shows that shifts in global climate can also destabilize modern-day societies.

Columbia University researchers have found that an El Niño, which brings hot, dry weather, doubles the likelihood of a civil war in more than 90 tropical countries affected by this climate cycle.

The authors, who are publishing their study in Nature on Thursday, say that El Niño may have played a role in one in five civil conflicts since 1950.

"We're extending the general hypothesis that throughout history human societies used to be influenced by the climate to say that, now, modern society continues to be influenced by the modern climate .... It wasn't so difficult to convince people that maybe Angkor Wat or Mayan civilizations collapsed due to climatic changes, but I think it's harder for people to accept that we still depend on the climate to a very large extent," author Solomon M. Hsiang, who is now at Princeton, said. (Disclosure: I know Hsiang from a class I took at Columbia University for my master's four years ago; he was the teaching assistant.)

While Hsiang and his co-authors emphasize their study focuses on the El Niño climate pattern, not climate change, their work could shed light on the potential consequences of global warming, which will make the world more El Niño-like: hotter and drier around the middle.

The connection between El Niño and warfare

El Niño and La Niña are two sides of a weather pattern that oscillates every three to seven years. During El Niño years, tropical parts of the globe become hotter and thirstier than they are during comparatively cool, rainy La Niña years.

The researchers found that the likelihood that a country in the tropics erupts into a civil conflict is 3% during a La Niña year, but 6% during an El Niño year. Countries whose climates are largely unaffected by El Niño only had a 2% chance of experiencing a civil conflict no matter the year.

Hsiang and his co-authors, Kyle C. Meng of Columbia and Mark Cane of Columbia's Lamont-Doherty Earth Observatory, did not analyze international conflicts because civil wars constitute the vast majority of all conflicts since 1950.

The map below shows, in red, the 93 countries that become warmer in El Niño years, and the 82 unaffected countries in blue.

While Hsiang does not say that El Niño causes these conflicts, he asserts it creates the conditions that make warfare more likely. He compared the presence of El Niño during a conflict to the presence of ice on a road during a car accident:

When there is more ice on the road, there are clearly more car accidents .... Does the ice cause car accidents? The truth is that drivers and their mistakes create car accidents, and the ice is not necessarily at fault, but the ice increases the likelihood with which drivers will make errors.

He, Meng and Cane speculate that the hotter, drier conditions of an El Niño indirectly lead to conflict in several ways: drought could cause large crop losses, which would in turn increase food prices and spark unemployment -- and the unemployed have more time to contemplate injustices and act on them. The researchers also cite evidence that hotter temperatures have physiological effects on people, making them more likely to instigate violence.

Import for climate change

Although this study did not look directly at the consequences of global warming, it could portend its impacts and be used to improve preparedness for humanitarian crises, because strong El Niños, which have the greatest connection to warfare, can be predicted up to two years in advance.

Hsiang, Meng and Cane learned from and built upon a widely criticized 2009 study by Marshall B. Burke and others that examined the effect of local climate cycles on conflict in Africa. In anticipation of similar attacks, Hsiang's team controlled for factors such as country's income, level of democracy, population, etc. They ran all the checks that the Burke study ran, as well as all the checks that a subsequent opposing study ran. Even then, the El Niño influence persisted.

Hsiang says that Burke's study tried to see what might happen in the future climate of individual countries by looking at local weather and rainfall -- i.e., if a country became slightly hotter than its neighbors, would it be more likely to have a civil conflict?

"But local weather and local rainfall are not good approximations of global climate change," he says, noting that global climate causes large patterns of environmental changes. For instance, a spate of recent related weather disasters has led to a spike in food prices. "When an El Niño event occurs, it leads to warming and drying throughout the entire tropics. It leads to a reduction in agricultural output throughout the entire tropics. And it is very likely that ... each individual society is affected ... when agricultural production is falling everywhere at the same time."

Cane says:

We can’t say what will happen for sure with climate change, but it does show beyond a doubt that even in the modern world, climate variations have an impact on people’s propensity to fight. It is difficult to see why that wouldn’t carry over into a world disrupted by global warming.